Free tool for freelancers and small businesses with overdue invoices
Late invoice interest and fee calculator
Interest = invoice amount × yearly rate × days late ÷ 365; amount due = invoice + interest + flat late fee. In the example, a $2,400 invoice 45 days late at 12% a year carries $35.51 of interest; with a $25 late fee the amount due is $2,460.51. 12% a year is the same as 1% a month.
Last updated 2026-11-12.
- Amount due now
- Interest so far
- Interest per day
- Same rate per month
Check the inputs: an amount above 0, a yearly rate between 0% and 100%, days and fee not negative.
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Example scenarios
Three sets of example inputs worked through the same formulas as the calculator above. They are illustrations, not averages or statistics.
| Scenario | Inputs | Interest | Late fee | Amount due | Interest per day |
|---|---|---|---|---|---|
| Low | $800.00 invoice, 8% a year, 15 days late | $2.63 | $0.00 | $802.63 | $0.18 |
| Typical | $2,400.00 invoice, 12% a year, 45 days late | $35.51 | $25.00 | $2,460.51 | $0.79 |
| High | $9,500.00 invoice, 18% a year, 90 days late | $421.64 | $50.00 | $9,971.64 | $4.68 |
Method: Interest = invoice amount × yearly rate × days late ÷ 365 (simple interest); amount due = invoice + interest + flat late fee. Check what your contract and local law allow before you charge either. Figures checked against the same formulas on 2026-11-12.
How the calculator works
Enter the invoice amount, the yearly interest rate your terms name, how many days have passed since the due date and any flat late fee. The calculator uses simple interest on actual days: the amount × the yearly rate ÷ 365 is the interest for one day, and the days late multiply it. The flat fee is added once. Many invoices state the rate per month instead; divide a yearly rate by 12 to compare, so 1.5% a month is 18% a year.
It does not compound: interest is never charged on interest. That is how most invoice terms are written, and it keeps the number easy for the client to check.
Worked example
A $2,400 invoice was due 45 days ago and the terms say 12% a year. One day of interest is $2,400 × 0.12 ÷ 365 = $0.79, so 45 days come to $2,400 × 0.12 × 45 ÷ 365 = $35.51. The terms also name a $25 late fee, so the amount due today is $2,400 + $35.51 + $25 = $2,460.51.
Put the calculation on the reminder: the original amount, the days late, the rate, the interest and the fee, each on its own line. A client who can check the number is more likely to pay it than to argue about it.
Before you charge interest or a fee
Charge only what you agreed in writing before the work, in the contract or on the invoice, and check your local law: many places cap late fees or interest, some set a statutory rate for business invoices, and consumer rules are stricter than business ones. If your terms say nothing, a friendly reminder usually comes first; adding interest the client never agreed to can start an argument instead of a payment.
What this tool does not do
It works out one invoice at a time and keeps nothing after you close the page. It does not send reminders, compound interest, convert currencies or tell you what your law allows; it is arithmetic on your numbers, not legal or tax advice.
Questions
Is 1.5% a month the same as 18% a year?
As simple interest, yes: 1.5 × 12 = 18. Enter 18 as the yearly rate.
Do I count from the invoice date or the due date?
From the due date: an invoice is late only after it is due. Enter the days since that date.
Does this page store my invoices?
No. The math runs in your browser; nothing is sent or saved.
Made by Bardach Technologies. We also make Clientsroster, a separate Notion template for solo freelancers that keeps leads, clients, projects, invoices and notes in six linked databases and tracks which invoices are due, paid or overdue (USD 59.99, one-time purchase on Gumroad). It does not send invoices or charge interest. This calculator is free and needs no sign-up; nothing is charged on this page. Get Clientsroster on Gumroad (USD 59.99)